Discrete resistors that require mounting directly to the surface of a circuit board are widely used in the electronics industry. A disadvantage with discrete resistors is the additional weight, assembly complexity and reduced circuit density that are incurred with their use. While the size of discrete resistors has been considerably reduced, with chip resistors commercially available having dimensions of as little as about 20.times.40 mils (about 0.5.times.1 millimeter), further miniaturization can generally be cost prohibitive for boards requiring numerous resistors.
Thick-film resistors are available alternatives to the use of discrete resistors. However, thick-film resistors have limited applications because of the size and tolerance control of the printing process used to form such resistors, which generally entails screen printing a thick-film resistive paste or ink on a substrate, such as a printed circuit board (PCB), flexible circuit, or a ceramic or silicon substrate. Thick-film inks are typically composed of an electrically-resistive material dispersed in an organic vehicle or polymer matrix material. After printing, the thick-film ink is typically heated to convert the ink into a suitable film that adheres to the substrate. If a polymer thick-film ink is used, the heating step serves to dry and cure the polymer matrix material. Other thick-film inks must be sintered, or fired, during which the ink is heated to burn off the organic vehicle and fuse the remaining solid material.
In addition to the size and tolerance limitations noted above, the use of a thick-film resistor is further complicated by the predictability and variability (or tolerance) of the electrical resistance, which are dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor terminations. Conventional thick-film resistors are rectangular shaped, with "x," "y" and "Z" dimensions corresponding to the electrical width (W), electrical length (L) and thickness, respectively, of the resistor. Control of the "x" and "y" dimensions of a thick-film resistor is particularly challenging in view of the techniques employed to print thick-film inks and the dimensional changes that may occur during subsequent processing. For rectangular screen-printed resistors, the x and z dimensions are determined by the resistor screening process, and the y dimension is determined by the termination pattern. Conventional screen printing techniques generally employ a template with apertures bearing the positive image of the resistor to be created. The template, referred to as a screening mask, is placed above and in close proximity to the surface of the substrate on which the resistor is to be formed. The mask is then loaded with the resistive ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate.
Compared to many other deposition processes, screen printing is a relatively imprecise process. Screen printed thick-film resistors having adequate tolerances in the x and y dimensions are often larger than chip resistors. Resistance value predictability is generally low, and precise tolerances typically cannot be maintained at aspect ratios (L/W) below 0.5 squares. As a result, one ink of a given resistivity, requiring one screening, cure and associated process steps, is required for each decade of resistance value needed in a circuit design, which often necessitates the use of three to four inks to complete one circuit. While resistance tolerances can be improved by laser trimming, such an operation is usually cost-prohibitive for complex circuits. As a result, screen printed thick-film resistors have found only limited application as a substitute for discrete resistors.
Accordingly, a need exists for a method for fabricating a thick-film resistor having more precise dimensional tolerances and a wider range of readily-producible resistances per single resistive ink than present thick-film resistors offer.